What happened

Pfizer (NYSE: PFE) announced Monday that its vaccine candidate BNT162b2 appears to be even more effective at preventing COVID-19 than anyone had hoped -- on the order of 90%! Stock markets loved that news, and today the Dow Jones Industrial Average is soaring nearly 5% as investors place their bets on a recovery.

But not all stocks are faring so well. It looks like shares of Stamps.com (NASDAQ:STMP), iRobot (NASDAQ:IRBT), and 2U (NASDAQ:TWOU) are getting simply devastated by today's news, down 9.9%, 10.5%, and 16.2%, respectively, in 12:45 p.m. EST trading.

Cartoon characters confused by stock chart arrow falling and crashing into floor

Image source: Getty Images.

So what

Why are these stocks down so much when seemingly everything else is up? To understand what's going on, consider the businesses they're in, how those businesses benefited from the pandemic and widespread stay-at-home orders ... and what's likely to happen when those stay-at-home orders go away and everything gets back to normal.

Stamps.com is most closely associated with the sale of postage over the internet, especially to small businesses and solo operators selling and mailing stuff from their homes. This is an area of the economy that flourished despite the recession -- sales were up 42% last quarter alone -- because so many people, having lost their day jobs, started new careers working from home.

One notable segment of the workforce impacted by the pandemic was housekeeping service providers, as homeowners sought safer alternatives to allowing other people into their homes to clean. iRobot shares benefited mightily -- and sales were up 43% last quarter -- as they bought up Roomba vacuums and Braava mops.

Finally, 2U sales boomed (a bit less than the others, up only 31% last quarter) as colleges scrambled to find ways to put their courses online during the pandemic. It's not necessarily a "work from home" stock, but it's clearly a beneficiary of pandemic fears regardless.

Now what

Now what happens if those concerns go away? What happens when a coronavirus vaccine that's 90% effective at preventing COVID-19 infections gets widely distributed so that (a) employers begin to rehire their employees, (b) homeowners feel comfortable allowing others to enter their homes to clean, and (c) colleges reopen to fully in-person education?

At the very least, growth rates are going to slow at Stamps.com, iRobot, and 2U. Sales will slow, and could even decline, as customers return to the ways they used to do things and abandon the alternatives and stopgaps that -- while useful and indeed critical in enabling them to survive the pandemic -- are no longer so essential in its aftermath.

That's what investors are afraid of. That's why Stamps, iRobot, and 2U stocks are all down today.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.